New fast-track definition proposal: _diffrn_radiation_wavelength_nominal

[Brian H. Toby]

I disagree. Cu K alpha 1 has been calibrated directly from primary  
standards. Anything done at a synchrotron or neutron source, however,...

On Nov 4, 2009, at 11:15 AM, kaduk at polycrystallography.com wrote:

> While I am sympathetic to the rationale, I'm not sure that this
> suggestion accomplishes very much.  The lattice parameters of the
> standard material depend on the wavelength NIST (or somebody else)
> used to determine them.  Perhaps all of the wavelengths in normal
> crystallographic work are "nominal"?
>
> Jim Kaduk
>
> Quoting James Hester <jamesrhester at gmail.com>:
>
>> Dear COMCIFS members: the following fast track proposal for a minor
>> change to the coreCIF dictionary has been approved by the core
>> Dictionary Maintenance Group after a 6-week comment period.  As per
>> the recently approved fast track process, it is now open for comments
>> from COMCIFS for a further 6 week period.
>>
>> James.
>> --------------------------------------------------------------------- 
>> -------------------------
>>
>> 2009-08-23 proposed by James Hester.
>> 2009-10-16 approved by the core Dictionary Maintenance Group under  
>> a fast
>>           track approval process.
>>
>> Explanation
>> ===========
>> The wavelength used for a measurement defines the
>>  length scales used in the crystallographic unit cell, so it is
>>  important that the quality of the number used for wavelength is
>>  understood.  When the bulk of experiments came from lab X-ray  
>> sources,
>>  the wavelength value was an accurate and precise quantity that could
>>  be relied on.  In contrast, synchrotron and neutron sources have
>>  comparatively poorly-defined wavelengths, subject to systematic  
>> errors
>>  (for example, changes in the monochromator d-spacing due to
>>  temperature effects, and angular offset errors).  Many facilities do
>>  not attempt to characterise the accuracy of such derived wavelength
>>  values. Instead users may (or may not) refine against a standard
>>  compound in order to recover an accurate wavelength value. I  
>> therefore
>>  propose a separate tag for those wavelength values that are nominal,
>>  that is, derived from positioning of optical elements rather than
>>  proper standards.  This removes ambiguity in determining whether or
>>  not a wavelength has been obtained from refinement against a  
>> standard,
>>  or is simply a nominal value based on beamline optics.
>>
>>
>> A new DDL1 item
>>
>>  data_diffrn_radiation_wavelength_nominal
>>     _name                      '_diffrn_radiation_wavelength_nominal'
>>     _category                    diffrn_radiation_wavelength
>>     _type                        numb
>>     _list                        both
>>     _list_reference            '_diffrn_radiation_wavelength_id'
>>      _enumeration_range           0.0:
>>     _units                       A
>>     _units_detail              'angstroms'
>>     _definition
>> ;              The incident radiation wavelength in Angstroms
>>               calculated from secondary information,
>>               for example monochromator angle or time of flight.
>>               If the wavelength has been determined
>>               using some type of standard,
>>               _diffrn_radiation_wavelength should be used
>> ;
>>
>> A change in definition of an existing item.  The current
>> definition reads 'The radiation wavelength in angstroms'.
>>
>>
>>  data_diffrn_radiation_wavelength
>>     _name                      '_diffrn_radiation_wavelength'
>>     _category                    diffrn_radiation_wavelength
>>     _type                        numb
>>     _list                        both
>>     _list_reference            '_diffrn_radiation_wavelength_id'
>>     _enumeration_range           0.0:
>>     _units                       A
>>     _units_detail              'angstroms'
>>     _definition
>> ;              The radiation wavelength in angstroms as
>>               determined from measurements using standards,
>>               for example an X-ray emission line,
>>               or when a refinement based on a standard
>>               material has been carried out.
>>               The details of such a refinement should be
>>               recorded in the _diffrn_radiation_special_details
>>               item.
>>  ;
>>
>> --
>> T +61 (02) 9717 9907
>> F +61 (02) 9717 3145
>> M +61 (04) 0249 4148
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>>
>
>
>
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Brian H. Toby, Ph.D.                            office: 630-252-5488
Senior Physicist/Materials Characterization Group Leader
Advanced Photon Source
9700 S. Cass Ave, Bldg. 433/D003             work cell: 630-327-8426
Argonne National Laboratory         secretary (Marija): 630-252-5453
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